Nickel cobalt phosphorus electroplating composition and its use in surface treatment of a workpiece

ABSTRACT

A nickel cobalt phosphorus electroplating composition includes a nickel salt, a cobalt salt, a phosphite-containing compound, and a multidentate chelating agent selected from triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof. An electroplating solution including the nickel cobalt phosphorus electroplating composition and a method for treating a surface of a workpiece using the electroplating solution are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Application No. 095117848, filed on May 19, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electroplating composition, more particularly to a nickel cobalt phosphorus electroplating composition. This invention also relates to an electroplating solution including the nickel cobalt phosphorus electroplating composition, and a method for treating a surface of a workpiece using the electroplating solution.

2. Description of the Related Art

It is known in the art to electroplate a surface of a substrate according to its intended use, e.g. electroplating with noble metals for increasing surface brightness, electroplating with chromium for increasing surface hardness and abrasion-resistance, electroplating with tin and lead for increasing corrosion-resistance, and electroplating with silver or copper for increasing conductivity. Particularly, a chromium electrodeposit has good hardness, corrosion-resistance and abrasion-resistance and is widely applied to surface treatments. However, since the electroplating solution used in chromium electroplating is poisonous, and since it is difficult to handle chromic acid-containing waste solution produced therefrom, chromium electroplating has incurred environmental concerns and import of chromium-electroplated products have been prohibited by several European countries.

Additionally, since a nickel electrodeposit has good brightness, abrasion-resistance, and corrosion-resistance, a nickel-electroplating system is also widely used in surface treatment. However, since hardness of a pure nickel electrodeposit is poor, it is improved by incorporating with other metals so as to form a nickel alloy deposit. For example, nickel cobalt (Ni/Co) alloy deposit is frequently applied to molding tools and punching-resistant cutting tools, such as formation of a surface of a female mold for compact disk manufacture, and formation of a surface of a copper mold for a continuous plating apparatus, due to its abrasion-resistance and high hardness. A deposit made from nickel-tungsten (Ni/W) alloy, nickel-manganese (Ni/Mn) alloy, nickel-phosphorus (Ni/P) alloy or nickel-iron (Ni/Fe) alloy may be used, in addition to nickel cobalt (Ni/Co) alloy. Even though an alloy deposit has a better strength and hardness than a pure metal deposit, the alloy deposit faces problems of internal stress and crispness.

U.S. Pat. No. 6,099,624 (hereinafter referred to as the '624 patent) discloses nickel-phosphorus alloy coatings produced by an electroplating bath including nickel alkane sulfonate, such as nickel methane sulfonate. The nickel methane sulfonate is dissociated into Ni(CH₃SO₃)⁺ ions in the electroplating bath and moves to the cathode during plating. Subsequently, Ni²⁺ ions are dissociated from Ni(CH₃SO₃)⁺ and are deposited on the cathode. The (CH₃SO₃)²⁻ ions thus formed are repelled by the negative cathode, which results in no polarization. Therefore, the cathode depositing efficiency carried out in the Examples of the '624 patent is no more than 80% and the nickel-phosphorus alloy coatings thus formed are crispy. In addition, it is stated in the '624 patent that other metals can be added into the electroplating bath so as to form a tri-component alloy coating. However, since each of the components in the alloy coating is independently chelated, inclusion of additional metal in the electroplating bath, such as cobalt, tends to result in uneven deposition through high and low current areas in the electroplating bath. Besides, addition of other metals into the electroplating bath results in changes in charge balance of the electroplating bath and properties of the alloy coating thus formed. Therefore, when an additional metal is added into the electroplating bath, other parameters such as ion concentrations, bath temperature, bath constitution, and pH value of the electroplating bath, and current density applied during electroplating need to be readjusted so as to form a desired multi-metal alloy coating. In addition, addition of other metals can aggravate internal stress and crispness problems of the alloy coating. Particularly, Co²⁺, Co³⁺, phosphite, and hypophosphite ions have a small solubility product constant in the electroplating bath and are easily precipitated out, which can result in an increase in the production cost for forming the multi-component alloy coating.

U.S. Pat. No. 6,406,611 (hereinafter referred to as the '611 patent) discloses an electroplating bath for electrodepositing a nickel cobalt phosphorus alloy. The electroplating bath includes nickel sulfate, cobalt sulfate, hypophosphorous acid or salt thereof, boric acid or a salt thereof, a monodentate organic acid or salt thereof, and a multidentate organic acid or a salt thereof. The hypophosphorous acid or a salt thereof is used as a phosphorus source for the electroplating bath and the pH value of the electroplating bath is controlled to be within a range of from 3 to 4.5. Phosphite ions and cobalt ions (Co²⁺ or Co³⁺) have a solubility product constant less than 10⁻⁶ at pH 4 and are easily precipitated out, thereby resulting in formation of colloids of cobalt phosphite. Consequently, the electroplating bath is unstable and is difficult to operate. Moreover, undesired suspended substances are formed in the electroplating bath, which can result in failure of electroplating in the electroplating cell.

Additionally, in the '611 patent, sodium hypophosphite is used as a phosphorous source of the nickel cobalt phosphorus alloy electrodeposit. The sodium ions released from the sodium hypophosphite result in excess hydrogen free radicals. The released hydrogen free radicals will permeate into the nickel seed and result in hydrogen embrittlement of the nickel cobalt phosphorus alloy electrodeposit. Additionally, in the electroplating bath of the '611 patent, an organic acid, such as glycolic acid and malic acid, is used as the multidentate chelating agent. However, since chelating ability of the organic acid with the nickel ion is higher than the cobalt ion, polarization of the nickel ion is higher than that of the cobalt ion. Hence, the cobalt content of the nickel cobalt phosphorus alloy electrodeposit formed in the low current area is higher than the cobalt content of the nickel cobalt phosphorus alloy electrodeposit formed in the high current area. The difference in cobalt and nickel metal contents for the nickel cobalt phosphorus alloy electrodeposits respectively formed in the low and high current areas, results in uneven hardness, internal battery effect, and a serious internal stress problem for the electrodeposits thus formed.

Therefore, there is still a need in the art to provide a nickel cobalt phosphorus electroplating composition suitable for forming a nickel cobalt phosphorus electrodeposit having low internal stress, good hardness and corrosion-resistance at an economical cost.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a nickel cobalt phosphorus electroplating composition that can overcome at least one of the aforesaid drawbacks associated with the prior art.

According to one aspect of the present invention, there is provided a nickel cobalt phosphorus electroplating composition comprising a nickel salt, a cobalt salt, a phosphite-containing compound, and a multidentate chelating agent selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof.

According to another aspect of the present invention, there is provided an electroplating solution including a nickel cobalt phosphorus electroplating composition and water and having a pH value ranging from 0.2 to 5. The nickel cobalt phosphorous electroplating composition includes a nickel salt dissolved in the water to form nickel ions; a cobalt salt dissolved in the water to form cobalt ions; a phosphite-containing compound dissolved in the water; and a multidentate chelating agent dissolved in the water and selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof.

According to yet another aspect of the present invention, there is provided a method for treating a surface of a workpiece. The method comprises placing the workpiece in an electroplating solution including a nickel cobalt phosphorus electroplating composition, and electroplating the workpiece in the electroplating solution under a current density so as to form a nickel cobalt phosphorus electrodeposit on the surface of the workpiece. The nickel cobalt phosphorus electroplating composition includes a nickel salt, a cobalt salt, a phosphite-containing compound, and a multidentate chelating agent selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which:

FIG. 1 is a threshold current vs. applied voltage plot to illustrate changes in the threshold current during the formation of nickel cobalt phosphorus electrodeposits from the preferred embodiment of a nickel cobalt phosphorus electroplating composition according to the present invention and a control group, respectively; and

FIG. 2 is a current efficiency vs. applied voltage plot to illustrate changes in the threshold current during the formation of nickel cobalt phosphorus electrodeposits from the preferred embodiment of a nickel cobalt phosphorus electroplating composition according to the present invention and a control group, respectively.

DETAILED DESCRIPTION OF TEE PREFERRED EMBODIMENTS

The preferred embodiment of a nickel cobalt phosphorus electroplating composition according to the present invention includes a nickel salt, a cobalt salt, a phosphite-containing compound, and a multidentate chelating agent selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof. Preferably, the nickel cobalt phosphorus electrodeposit formed from the nickel cobalt phosphorus electroplating composition has 68-5 to 94.5% by weight of nickel, 5 to 15.5% by weight of cobalt, and 0.5 to 16% by weight of phosphorus, based on total weight of the nickel cobalt phosphorus electrodeposit.

More preferably, the multidentate chelating agent included in the nickel cobalt phosphorus electroplating composition is triethylene tetraamine.

In another preferred embodiment, the phosphite-containing compound included in the nickel cobalt phosphorus electroplating composition is a sodium-free phosphite-containing compound selected from the group consisting of phosphorous acid, nickel phosphite, cobalt phosphite and combinations thereof. More preferably, the phosphite-containing compound is phosphorous acid.

In yet another preferred embodiment, the nickel salt included in the nickel cobalt phosphorus electroplating composition is selected from the group consisting of nickel carbonate, nickel hydroxide, nickel oxide, and combinations thereof. More preferably, the nickel salt is a combination of nickel carbonate and nickel hydroxide.

In still another preferred embodiment, the cobalt salt included in the nickel cobalt phosphorus electroplating composition is selected from the group consisting of cobalt carbonate, cobalt hydroxide, cobalt oxide, and combinations thereof.

The preferred embodiment of the electroplating solution according to this invention includes the nickel cobalt phosphorus electroplating composition as mentioned above and water. Preferably, the electroplating solution has a pH value ranging from 0.2 to 5. More preferably, the pH value of the electroplating solution ranges from 1.2 to 2. Most preferably, the pH value of the electroplating solution ranges from 1.5 to 1.9. The nickel cobalt phosphorus electroplating composition is dissolved in the water to form nickel ion, cobalt ions, and phosphite ions in the electroplating solution.

In another preferred embodiment, the electroplating solution further includes an electrolyte selected from the group consisting of phosphoric acid, sulfuric acid, hydrochloride, and combinations thereof.

Preferably, the concentration of the nickel ions ranges from 20 to 100 g/l, the concentration of the cobalt ions ranges from 0.5 to 15 g/l, the concentration of the phosphite ions ranges from 5 to 80 g/l, the concentration of the electrolyte ranges from 20 to 200 g/l, and the concentration of the multidentate chelating agent ranges from 20 to 200 g/l. More preferably, the concentration of the nickel ions ranges from 40 to 70 g/l, the concentration of the cobalt ions ranges from 4 to 7 g/l, the concentration of the phosphite ions ranges from 20 to 40 g/l, the concentration of the electrolyte ranges from 100 to 140 g/l, and the concentration of the multidentate chelating agent ranges from 60 to 120 g/l. Most preferably, the concentration of the nickel ions is 55 g/l, the concentration of the cobalt ions is 5.5 g/l, the concentration of the phosphite ions is 30 g/l, the concentration of the electrolyte is 120 g/l, and the concentration of the multidentate chelating agent is 90 g/l.

In another preferred embodiment, the electrolyte included in the electroplating solution is phosphoric acid. In yet another preferred embodiment, the multidentate chelating agent included in the electroplating solution is triethylene tetraamine.

It should be noted that when the electroplating solution has more than 200 g/l of the multidentate chelating agent, the cathode efficiency decreases, whereas when the electroplating solution has less than 20 g/l of the multidentate chelating agent, difference in ion moving rates between nickel ions and cobalt ions in the electroplating solution is undesirably enlarged.

When the electroplating solution has more than 80 g/l of the phosphite ions, the nickel cobalt phosphorus electrodeposit thus made is too crispy, whereas when the electroplating solution has less than 5 g/l of the phosphite ions, the nickel cobalt phosphorus electrodeposit thus made has poor hardness.

When the electroplating solution has more than 100 g/l of the nickel ions, the nickel cobalt phosphorus electrodeposit thus made has poor hardness, whereas when the electroplating solution has less than 20 g/l of the nickel ions, excess amount of the cobalt ions is required and the production cost is increased.

When the electroplating solution has more than 15 g/l of the cobalt ions, excess amount of the cobalt ions is required and the production cost is increased, whereas when the electroplating solution has less than 0.5 g/l of the cobalt ions, the nickel cobalt phosphorus electrodeposit thus made has poor hardness.

In addition, any additive suitable for use in the electroplating solution of this invention can be added thereto. These additives are known in the art and can be properly selected by the skilled artisan. Non-limiting examples of the additives include a brightener for enhancing reflection property of the nickel cobalt phosphorus electrodeposit, a flattening agent for enhancing smoothness of the nickel cobalt phosphorus electrodeposit, and a wetting agent.

The preferred embodiment of a method for treating a surface of a workpiece according to this invention includes placing the workpiece in an electroplating solution as mentioned above, and electroplating the workpiece in the electroplating solution under a current density so as to form a nickel cobalt phosphorus electrodeposit on the surface of the workpiece.

In one preferred embodiment, the electroplating solution is maintained at a temperature ranging from 40° C. to 70° C. during the electroplating of the workpiece in the electroplating solution. More preferably, the electroplating solution is maintained at a temperature ranging from 50° C. to 60° C.

In another preferred embodiment, the current density ranges from 0.5 to 10 A/dm² during the electroplating of the workpiece in the electroplating solution. More preferably, the current density ranges from 1.5 to 6 A/dm².

In yet another preferred embodiment, the electroplating of the workpiece in the electroplating solution is conducted using an undissolvable anode, such that undesired ions dissociated from the anode, disruption of ion balance of the electroplating solution, and shorting of the electroplating solution can be avoided. More preferably, the undissolvable anode is made from platinum titanium mesh.

In still another preferred embodiment, an ion-concentration modifier is added to the electroplating solution during the electroplating of the workpiece in the electroplating solution so as to maintain the ion concentrations in the electroplating solution to be within the required range. Non-limiting examples of the ion-concentration modifier include nickel carbonate, nickel hydroxide, nickel oxide, cobalt carbonate, cobalt hydroxide, cobalt oxide, phosphorous acid and the like.

Alternatively, after the electroplating of the workpiece in the electroplating solution, the electroplated workpiece is hot-worked. Preferably, the hot-working of the electroplated workpiece is conducted at a temperature ranging from 200° C. to 450° C.

It is noted that a nickel cobalt phosphorus electrodeposit is formed on the surface of the workpiece by the method this invention. In particular, the nickel cobalt phosphorus electrodeposit thus formed contains 68.5 to 94.5% by weight of nickel, 5 to 15.5% by weight of cobalt, and 0.5 to 16% by weight of phosphorus, based on total weight of the nickel cobalt phosphorus electrodeposit. It has been demonstrated by the inventor(s) that the nickel cobalt phosphorus electrodeposit formed on the workpiece has excellent corrosion-resistance when having 81% weight of nickel and 6% by weight of cobalt, and that the nickel cobalt phosphorus electrodeposit has an optimum hardness of up to about 1050 Hv, when having 80% weight of nickel and 11% by weight of cobalt.

In addition, in the absence of hot-working treatment, the nickel cobalt phosphorus electrodeposit formed on the workpiece has a face-centered cubic (FCC) crystal form and includes a nickel cobalt solid-solution, an amorphous nickel cobalt alloy (r form), and phosphorus which is doped in grain boundary in the nickel cobalt solid-solution or dispersed in the amorphous nickel cobalt alloy. After hot-working at a temperature of about 400° C., the nickel cobalt phosphorus electrodeposit is decomposed into Ni₃P and Co₃P body-centered tetragonal (BCT) crystal forms in a parallel arrangement. The hot-worked nickel cobalt phosphorus electrodeposit thus formed includes a first solid solution containing nickel and cobalt and having a FCC crystal form, a second solid solution containing Ni₃P and CO₃P and having a BCT crystal form, and an amorphous intermetallic compound including at least two elements selected from the group consisting of nickel, cobalt and phosphorus, and dispersed in grain boundary between the first and second solid solutions.

The nickel cobalt phosphorus electrodeposit formed according to the method of the present invention, regardless of whether or not it is subjected to hot-working, has physical and chemical properties superior over those of the conventional alloy coatings.

Particularly, the hot-worked nickel cobalt phosphorus electrodeposit thus formed has a reflectivity comparable with a conventional nickel plating and ranging from 45% to 65%. The hot-worked nickel cobalt phosphorus electrodeposit thus formed also has a porosity in inverse proportion to the square of the layered thickness thereof. Particularly, the nickel cobalt phosphorus electrodeposit can be deemed pore-free when having a thickness of more than 30 μm. Besides, the nickel cobalt phosphorus electrodeposit has a density ranging from 8.2 to 8.4 g/cm³ without being hot-worked. After being hot-worked, the density will be slightly increased. The nickel cobalt phosphorus electrodeposit has a resistivity ranging from 70 to 85, μΩ-cm, and has a contact resistivity ranging from 25 to 35 mΩ-m under a contact pressure of about 1 N. The values of both resistivites are 10 times that of a pure nickel plating layer. Moreover, the nickel cobalt phosphorus electrodeposit has a thermal potential ranging from 0.5 to 0.1 g V/K, an electromagnetic shielding effectiveness equal to a tenth part of that of the pure nickel plating layer, and a thermal conductivity ranging from 4.5 to 5.5 W/m° C.

The layered thickness of the nickel cobalt phosphorus electrodeposit formed on the workpiece according to the method of the present invention can be thin or thick based on the actual requirements, such as that referred as an electroplated (thin plating layer) article or an electroformed (thick plating layer) article.

It is noted that measurement of the internal stress of the nickel cobalt phosphorus electrodeposit is conducted by allowing the nickel cobalt phosphorus electrodeposit to deform solely by the internal stress and then by applying a force (in a unit of kgf/mm²) that is sufficient to recover the initial shape thereof. A positive value for the applied force is an indication of tensile stress, whereas a negative value for the applied force is an indication of compressive stress.

The internal stress of the nickel cobalt phosphorus electrodeposit also varies with the materials used for the substrate on which the electrodeposit is formed. In the case of the electroplated article, when the substrate is made from steel, the internal stress of the nickel cobalt phosphorus electrodeposit without hot-working ranges from 2.5 to 3.5 kPa/mm²; when the substrate is made from an aluminum alloy, the internal stress of the nickel cobalt phosphorus electrodeposit without hot-working ranges from 7 to 10 kPa/mm², and when the substrate is made from copper, the internal stress of the nickel cobalt phosphorus electrodeposit ranges from 2 to 3 kPa/mm² without hot-working. Nevertheless, after the abovementioned the nickel cobalt phosphorus electrodeposit-deposited substrates are hot-worked, each of which has the internal stress ranging from −0.5 to 0.5 kPa/mm². In the case of the electroformed article, the internal stress of the nickel cobalt phosphorus electrodeposit ranges from 0.5 to 1 kPa/mm² in the absence of hot-working and ranges from −0.5 to 0.5 kPa/mm² with hot-working.

The modulus of elasticity of the nickel cobalt phosphorus electrodeposit formed according to this invention is five times of that of the electroless plating nickel and varies with the materials for the substrate on which the electrodeposit is formed. In the case of the electroplated article, the modulus of elasticity (elasticity coefficient) of the nickel cobalt phosphorus electrodeposit generally reaches about 200 Gpa/mm without hot-working. In the case of the electroformed article, the modulus of elasticity of the nickel cobalt phosphorus electrodeposit generally reaches about 277 Gpa/mm with hot-working. The nickel cobalt phosphorus electrodeposit formed according to this invention has a tensile strength which is two to four times of that of the electroless plating nickel and reaches 2,100 MPa, and an elongation which reaches 8%.

In yet another preferred embodiment, a tin plating layer may be formed on the nickel cobalt phosphorus electrodeposit formed according to this invention so as to strengthen the structural strength of the nickel cobalt phosphorus electrodeposit which can reach 650 MPa. Additionally, the nickel cobalt phosphorus electrodeposit formed according to this invention satisfies MILC-26074E, AMS2404B and AMS2405 standards, and is suitably applied to corrosion-resistant plated articles made from iron-copper alloys or copper alloys as a top or bottom protective coating.

EXAMPLE

Chemicals used for the Example

-   1. phosphoric acid: commercially available 85 phosphoric acid     solution -   2. Crystalline Phosphite: phosphite 8504, sodium hypophosphite 8467,     potassium phosphite 7544, potassium hypophosphite 7520, or calcium     phosphite 1674, commercially available from Merck -   3. Triethylenetetramine: Product no. 112-24-3, commercially     available from Aldrich -   4. Cobaltous carbonate: Product no. 2391, commercially available     from Merck -   5. Nickel carbonate hydroxide: Product no. 123987 A1, commercially     available from Japan Okuno Chemical Industries Co., Ltd. -   6. Sodium lauryl sulfonate: Product no. 151-21-3, commercially     available from Fluka -   7. 1-naphthol-4,6,9-trisulfonic acid, sodium salt: Product no 1873,     commercially available from Merck -   8. Coumarin 2543: commercially available from Merck     Apparatus used for the Examples -   1. Hull cell: commercially available from Jun-Guang Co., Ltd. -   2. Atomic absorption spectrophotometer: Model 906AA, commercially     available from GBC.     Preparation of the Electroplating Solution

Example 1

450 ml of water, 142 g of phosphoric acid, 30 g of phosphorous acid crystals, 20 g of cobalt carbonate, and 90 g of triethylene tetraamine were mixed together with stirring to form a mixture. Nickel cobalt was slowly added into the mixture until pH value of the mixture reached 1.9. After carbon dioxide gas generated during the mixing was dissipated from the mixture, 1 L of water was added into the mixture until the volume of the mixture reached 1 liter. The electroplating solution thus formed has a composition including 55 g/l of nickel ions, 5.5 g/l of cobalt ions, 120 g/l of phosphate ions, 30 g/l of phosphite ions and 90 g/l of triethylene tetraamine.

Comparative Example 1

450 ml of water, 142 g of phosphoric acid, 30 g of phosphorous acid crystals and 20 g of cobalt carbonate were mixed together with stirring to form a mixture. Nickel cobalt was slowly added into the mixture until pH value of the mixture reached 1.9. After carbon dioxide gas was dissipated from the mixture, 1 L of water was added into the mixture until the volume of the mixture reached 1 liter. The electroplating solution thus formed has a composition including 55 g/l of nickel ions, 5.5 μl of cobalt ions, 120 g/l of phosphate ions, and 30 g/l of phosphate ions.

Formation of Nickel Cobalt Phosphorus Electrodeposit Samples Using the Electroplating Solution of Example 1

Samples SE1 to SE6

20 ppm of sodium lauryl sulfonate and 2 g/l of 1-naphthol-4,6,9-trisulfonic acid sodium salt were added into the electroplating solution prepared in Example 1 so as to form the electroplating bath. The electroplating bath was introduced to the Hull cell. The Hull cell was operated under the following conditions: bath temperature: 55° C.; and applied a constant current: 10.0 A. A platinum titanium mesh and a 10 cm×5 cm copper substrate were placed in the Hull cell and were used as the anode and cathode, respectively. The experiment was run on an hourly basis for 160 hours. At the end of each hour, the copper substrate having the nickel cobalt phosphorus electrodeposit formed thereon was removed from the bath, and a fresh copper substrate was subsequently placed into the bath for the next hour deposition. The concentrations of nickel and cobalt in the electroplating solution were readjusted to their initial values at the beginning of each hour. Each deposited copper substrate was cut into four equal parts sequentially from one end thereof formed in the high current area to the other end thereof formed in the low current area. The four parts are respectively designated as high current part, middle-high current part, middle-low current part, and low current part. The weight of the nickel cobalt phosphorus electrodeposit on each cut part was measured. Each of the parts was cut into halves so as to form two pieces. One of the pieces of each part was dissolved in aqua regia and then analyzed by the atomic absorption spectrophotometer so as to determine the nickel and cobalt contents in the nickel cobalt phosphorus electrodeposit formed on each piece. The phosphorus content in the nickel cobalt phosphorous electrodeposit formed on each piece was subsequently obtained by subtraction. On the other hand, the other piece of each part was used for hardness test. The hardness of each of the other pieces was determined according to CNS 7094 Z8017 method before and after the hot-working operation, respectively. The hot-working operation of the other pieces was conducted at a temperature of 400° C. for one hour. Table 1 lists the measured data for Samples SE1 to SE6 that correspond to the nickel cobalt phosphorus electrodeposits formed at the 5^(th), 10^(th), 20^(th), 40^(th), 80^(th), and 160^(th) hours, respectively.

Formation of Nickel Cobalt Phosphorus Electrodeposit Samples Using the Electroplating Solution of Comparative Example 1

Samples CE1 to CE6

The nickel cobalt phosphorus electrodeposits of Samples CE1 to CE6 were formed in a manner similar to those of Samples SE1 to SE6 except that the electroplating solution of Comparative Example 1 was used. TABLE 1 Middle- Middle- High high low Low current current current current Sample Properties part part part part SE1 Weight of the NiCoP 3.21 2.67 2.15 1.70 electrodeposit (g) Ni wt %¹ 79.0 78.5 77.5 77.0 Co wt %² 12.6 12.3 11.7 11.4 P wt %³ 8.4 9.2 10.8 11.6 R_(Ni/Co) ⁴ 6.27 6.38 6.612 6.75 R_(mean) ⁵ 6.50 Hardness before 565 582 590 624 hot-working (Hv)⁶ Hardness after 1004 1036 1075 1103 hot-working (Hv)⁷ SE2 Weight of the NiCoP 3.27 2.61 1.94 1.48 electrodeposit (g) Ni wt %¹ 79.6 78.8 77.2 76.4 Co wt %² 12.8 12.4 11.6 11.2 P wt %³ 7.6 8.8 11.2 12.4 R_(Ni/Co) ⁴ 6.42 6.35 6.66 6.82 R_(mean) ⁵ 6.56 Hardness before 573 590 598 616 hot-working (Hv)⁶ Hardness after 1010 1057 1089 1117 hot-working (Hv)⁷ SE3 Weight of the NiCoP 3.51 2.37 2.20 1.75 electrodeposit (g) Ni wt %¹ 79.6 78.8 77.2 76.4 Co wt %² 12.4 12.2 11.8 11.6 P wt %³ 8.1 9.0 11.0 11.9 R_(Ni/Co) ⁴ 6.42 6.46 6.54 6.59 R_(mean) ⁵ 6.50 Hardness before 573 576 586 606 hot-working (Hv)⁶ Hardness after 1034 1031 1088 1094 hot-working (Hv)⁷ SE4 Weight of the NiCoP 3.44 2.44 1.79 1.34 electrodeposit (g) Ni wt %¹ 79.9 79.0 77.0 76.1 Co wt %² 13.0 12.5 11.5 11.0 P wt %³ 7.1 8.6 11.4 12.9 R_(Ni/Co) ⁴ 6.15 6.32 6.70 6.92 R_(mean) ⁵ 6.52 Hardness before 578 594 583 624 hot-working (Hv)⁶ Hardness after 1027 1071 1097 1124 hot-working (Hv)⁷ SE5 Weight of the NiCoP 3.66 2.22 1.83 1.37 electrodeposit (g) Ni wt %¹ 79.9 79.0 77.0 76.1 Co wt %² 12.6 12.3 11.7 11.4 P wt %³ 7.4 8.7 11.3 12.6 R_(Ni/Co) ⁴ 6.34 6.42 6.58 6.68 R_(mean) ⁵ 6.51 Hardness before 578 583 585 624 hot-working (Hv)⁶ Hardness after 1049 1068 1098 1106 hot-working (Hv)⁷ SE6 Weight of the NiCoP 3.55 2.33 2.12 1.67 electrodeposit (g) Ni wt %¹ 78.8 78.4 77.6 77.2 Co wt %² 12.3 12.2 11.8 11.7 P wt %³ 8.9 9.4 10.6 11.1 R_(Ni/Co) ⁴ 6.41 6.43 6.58 6.60 R_(mean) ⁵ 6.50 Hardness before 561 574 581 621 hot-working (Hv)⁶ Hardness after 1038 1038 1069 1090 hot-working (Hv)⁷

TABLE 2 Middle- Middle- High high low Low current current current current Sample Properties part part part part CE1 Weight of the NiCoP 3.49 2.39 2.09 1.63 electrodeposit (g) Ni wt %¹ 83.2 82.3 81.9 81.6 Co wt %² 10.3 10.1 9.9 9.7 P wt %³ 6.5 7.6 8.3 8.7 R_(Ni/Co) ⁴ 8.08 8.15 8.27 8.41 R_(mean) ⁵ 8.23 Hardness before 537 559 596 593 hot-working (Hv)⁶ Hardness after 913 909 914 954 hot-working (Hv)⁷ CE2 Weight of the NiCoP 3.62 2.26 1.96 1.50 electrodeposit (g) Ni wt %¹ 85.8 82.9 81.5 80.6 Co wt %² 10.00 9.60 9.50 8.80 P wt %³ 4.20 7.50 9.00 10.60 R_(Ni/Co) ⁴ 8.58 8.64 8.58 9.16 R_(mean) ⁵ 8.74 Hardness before 553 546 571 575 hot-working (Hv)⁶ Hardness after 940 936 946 928 hot-working (Hv)⁷ CE3 Weight of the NiCoP 3.28 2.60 2.11 1.66 electrodeposit (g) Ni wt %¹ 83.9 82.5 81.8 81.3 Co wt %² 10.4 10.2 9.8 9.6 P wt %³ 5.6 7.3 8.4 9.1 R_(Ni/Co) ⁴ 8.07 8.09 8.35 8.47 R_(mean) ⁵ 8.25 Hardness before 542 565 576 577 hot-working (Hv)⁶ Hardness after 873 905 924 966 hot-working (Hv)⁷ CE4 Weight of the NiCoP 3.66 2.22 1.88 1.43 electrodeposit (g) Ni wt %¹ 86.0 83.0 81.5 80.5 Co wt %² 10.6 10.4 9.6 9.2 P wt %³ 3.2 6.6 8.9 10.3 R_(Ni/Co) ⁴ 7.96 7.98 8.49 8.75 R_(mean) ⁵ 8.30 Hardness before 554 584 608 591 hot-working (Hv)⁶ Hardness after 948 950 949 1004 hot-working (Hv)⁷ CE5 Weight of the NiCoP 3.21 2.66 2.10 1.65 electrodeposit (g) Ni wt %¹ 84.9 82.7 81.6 80.9 Co wt %² 10.7 10.4 9.6 9.3 P wt %³ 4.4 6.9 8.7 9.8 R_(Ni/Co) ⁴ 7.93 7.95 8.50 8.70 R_(mean) ⁵ 8.27 Hardness before 548 581 585 608 hot-working (Hv)⁶ Hardness after 859 906 936 997 hot-working (Hv)⁷ CE6 Weight of the NiCoP 3.39 2.49 2.12 1.67 electrodeposit (g) Ni wt %¹ 86.1 83.0 81.5 80.4 Co wt %² 10.7 10.4 9.6 9.3 P wt %³ 3.1 6.6 8.9 10.3 R_(Ni/Co) ⁴ 8.05 7.98 8.49 8.65 R_(mean) ⁵ 8.29 Hardness before 555 582 560 604 hot-working (Hv)⁶ Hardness after 894 903 951 999 hot-working (Hv)⁷ ¹Nickel content in the corresponding region of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit ²Cobalt content in the corresponding region of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit ³Phosphorus content in the corresponding region of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit ⁴Ratio of nickel content to cobalt content in the corresponding region of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit ⁵Mean value of the R_(Ni/Co) values of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit ⁶Hardness of the piece determined before hot-working ⁷Hardness of the piece determined after hot-working

According to the results shown in Table 1 (for Samples SE1 to SE6), the Ni content, the Co content, and the phosphorus content in each of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit, and the total weight of the NiCoP electrodeposit have relatively small change for Samples SE1 to SE6, which is an indication of achieving stable electroplating. This stable condition is attributed to addition of nickel carbonate, cobalt carbonate, and phosphorus acid into the electroplating bath, which supplements the nickel, cobalt, and phosphorus ions in the bath during consumption of these ions and so as to maintain balance among these ions in the bath. In addition, since the electroplating bath according to this invention is acidic, the carbonate ions released into the electroplating bath will be converted into carbon dioxide gas dissipated therefrom so that the electroplating bath will not be adversely affected due to the presence of the carbonate ions.

In addition, in Samples SE1 to SE6, since triethylene tetraamine used as the chelating agent has a chelating ability with the nickel ions similar to that with the cobalt ions, the nickel and cobalt ions in the electroplating bath have similar ion mobility. The chelated nickel and cobalt ions require additional energy so as to dissociate, thereby facilitating reduction of the nickel and cobalt ions on the cathode.

Comparing the experimental data shown in Table 1 with that shown in Table 2, either the ratio value (R) of the nickel content to the cobalt content of each of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit of Samples SE1 to SE6 or the mean ratio value (R_(m)) of the nickel content to the cobalt content of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit of Samples SE1 to SE6 is lower than those of the Samples CE1 to CE6. This demonstrates that inclusion of triethylene tetraamine in the electroplating bath results in a decrease in the nickel content in the NiCoP electrodeposit and an increase in the cobalt content in the NiCoP electrodeposit. In addition, the phosphorus content in the NiCoP electrodeposit formed in Samples SEE to SE6 is higher than that of comparative Examples 2-7 by 1 to 2 wt %. Thus, the NiCoP electrodeposit of the Samples SE1 to SE6 has hardness higher than that of the NiCoP electrodeposit of the Samples CE1 to CE6. The effect of the increase in the cobalt content and the phosphorus content and the decrease in the nickel content may be attributed to polarization of the nickel and cobalt ions but no polarization of phosphate or phosphite ions. The increase in the phosphrous content increases the hardness of the electrodeposit thus formed.

In addition, since triethylene tetraamine is able to chelate with nickel and cobalt ions in an ion form of Ni_(x)Co_(3-x)(triethylene tetraamine)⁺⁶ (x is an integer ranging from 1 to 3), triethylene tetraamine is able to carry nickel and cobalt ions in a certain ratio and passes through a bipolar layer to the cathode. Particularly, the ratio values (R) of the nickel content to the cobalt content of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit for Samples CE1 to CE6 are substantially maintained at a relatively narrow range, whereas the ratio values (R) of the nickel content to the cobalt content of the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit for Comparative Examples 2 to 7 cannot maintain a narrow range.

In addition, inclusion of triethylene tetraamine in the electroplating bath results in a decrease in variation in the ratio values of nickel content: cobalt content: phosphorus content for the high current, middle-high current, middle-low current, and low current regions of the NiCoP electrodeposit. Consequently, the internal stress in the NiCoP electrodeposit is distributed more evenly than those of the Samples, and the problem met in the '611 patent can be overcome.

Corrosion-Resistance Comparison Between the NiCoP Electrodeposit Formed According to this Invention and the '611 patent

Copper substrates having a size of 10 cm×5 cm were individually electroplated in the electroplating bath of Example 1 for 25 minutes so as to form specimens 1, 2, and 3 according to this invention. Specimens 4, 5, 6 of the '611 patent were prepared in a manner similar to that of the specimen of this invention, except that the copper substrates were individually electroplated in an electroplating bath having a composition as described in Table 1 of Example 1 of the '611 patent. The specimens of this invention and the '611 patent were subjected to the copper-accelerated acetic acid salt spray (Fog) test (CASS Test) according to ASTM Test Method S368-61T in a standard ASTM B117 salt fog cabinet. The corrosion-resistance effect on the specimens for this invention and the '611 patent was evaluated in accordance with ASTM committee B-8 and the results are shown in Table 3. TABLE 3 CASS TEST Thickness of the NiCoP electrodeposit of Thickness of the NiCoP electrodeposit of Operation specimens of this invention (μm) specimens of the ′611 patent (μm) time (hr(s)) Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Specimen 6 0 20.5 20.9 19.6 20.1 21.2 20.4 12 10 10 10 10 10 9.5 24 10 10 10 9.5 10 9.5 36 10 10 10 9.5 10 9 48 10 10 10 9 9 8 60 9.5 10 10 8 8.5 7.5 72 8.5 10 9.5 7 7 6 96 8.5 10 9 5 6.5 4 108 8.5 10 9 2 4 1.5 120 8 9.5 9 0 1.5 0 240 7 8.5 8.5 — 0 —

According to the results shown in Table 3, it can be found that the NiCoP electrodeposit formed according to this invention has a much better corrosion-resistance than that of the NiCoP electrodeposit formed according to the '611 patent. The difference between the specimens of this invention and the specimens of the '611 patent in the corrosion-resistance of the NiCoP electrodeposit increases with the increase of the operation time. That is to say, the corrosion of the NiCoP electrodeposit formed according to the '611 patent worsens with the increase of the operation time.

In addition, the corrosion that occurred in the NiCoP electrodeposit formed according to this invention was pitting corrosion, whereas the corrosion that occurred in the NiCoP electrodeposit formed according to the '611 patent was scratch corrosion. This demonstrates that the adhesion of the NiCoP electrodeposit formed according to this invention to the copper substrate is stronger than that of the NiCoP electrodeposit formed according to the '611 patent to the copper substrate.

Hydrogen Over Voltage

A copper substrate having a size of 10 cm×5 cm was electroplated in the electroplating bath of Example 1 for 5 hours so as to form a specimen according to this invention. The specimen of the '611 patent was prepared in a manner similar to that of the specimen of this invention, except that the copper substrate was electroplated in an electroplating bath having a composition as described in Table 1 of Example 1 of the '611 patent. The specimens of this invention and the '611 patent were subjected to the current voltammetry test during the electroplating process. Results of threshold current analysis are shown in FIG. 1 and results of cathode current of ficiency analysis are shown in FIG. 2. The reference electrode was made from AgCl. The curve drawn from the diamond symbol “♦” stands for the results obtained from the specimen of this invention, and the curve drawn from the square symbol “▪” stands for the results obtained from the specimen of the '611 patent.

According to the results shown in FIG. 1, when the applied voltage was higher than 3.25 V, the threshold current of the specimen of this invention during the electroplating process did not increase with the increase in the applied voltage. Further referring to the results of the current efficiency analysis shown in FIG. 2, the cathode current efficiency of the specimen of this invention during the electroplating process was maintained at a value of about 100%, and hydrogen gas was not found during and after the electroplating process.

Referring back to the results shown in FIG. 1, when the applied voltage was higher than 3.25V, the threshold current of the specimen of the '611 patent during the electroplating process was significantly increased with the increase in the applied voltage. Further referring to the results of the current efficiency analysis shown in FIG. 2, the cathode current efficiency of the specimen of the '611 patent during the electroplating process decreased to a value of about 75%, and hydrogen gas was found during and after the electroplating process. Thus, this demonstrates that the composition of the electroplating bath of the '611 patent has a low hydrogen over voltage, which tends to result in reduction of hydrogen ion to hydrogen gas, which, in turn, results in hydrogen embrittlement phenomena. On the contrary, the composition of the electroplating bath according to this invention has no such problem.

Although this invention would not be bound by any theory, the reasons for avoiding hydrogen embrittlement for the composition of the electroplating bath according to this invention may be attributed to the neutral property of the triethylene tetraamine which is reduced after nickel and cobalt ions were reduced from Ni_(x-)CO_(3-x) (triethylene tetraamine)⁺⁶ (x is an integer ranging from 1 to 3) on the cathode. The reduced triethylene tetraamine is able to attract hydrogen ion (H⁺) and to be attached to a tip of the cathode, thereby increasing the hydrogen over voltage and generating polarizing effect. Therefore, the cathode current efficiency of the electroplating bath according to this invention is able to reach 100%. In addition, since triethylene tetraamine is able to attract hydrogen ions in the vicinity of the cathode and to be attached to the tip of the cathode, growth of the NiCoP electrodeposit having a needle or tumor shape can be avoided so as to provide a flatter electrodeposit.

In view of the foregoing, this invention provides a method for treating a surface of a workpiece involving the use of an undissolvable anode so as to avoid production of undesired ions, and addition of nickel carbonate, cobalt carbonate and phosphorous acid so as to supplement nickel, cobalt and phosphite ions of the electroplating bath. Therefore, a NiCoP electrodeposit having required and desired properties can be obtained. In addition, by-products produced during the electroplating process include phosphite ions and carbon dioxide gas, both of which have no adverse effect on ion balance of the electroplating bath, and thus, the electroplating bath of this invention is able to sustain a relatively long operation time.

Additionally, the NiCoP electrodeposit formed from the composition and the electroplating solution according to this invention has superior physical and chemical properties. For instance, the NiCoP electrodeposit formed according to this invention has a hardness value of up to 1000 Hv after being hot-worked, and such hardness value is much higher than that of the conventional hard chromium electroplating layer. In addition, the hardness and the toughness of the NiCoP electrodeposit formed according to this invention is comparable with 6W6 die steel.

Furthermore, according to the results of the above CASS test, the NiCoP electrodeposit formed according to this invention, having a thickness of 20 μm, is endurable for 240 hours when subjected to the CASS test. When the NiCoP electrodeposit is applied to a surface of an engine made from light-weight alloy such as aluminum or magnesium alloys, the engine will have superior hardness and toughness and consumption of petroleum can be reduced. In addition, since the NiCoP electrodeposit formed according to this invention has a smooth surface, consumption of lubricant can also be reduced. Therefore, the NiCoP electrodeposit formed from the electroplating composition and the electroplating solution according to this invention has low internal stress, high hardness, high corrosion-resistance, and surface smoothness. The method for treating a surface of a workpiece involving utilization of the electroplating composition and the electroplating solution according to this invention is useful for replacing the highly pollutive conventional hard chromium surface treatment techniques.

While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements. 

1. A nickel cobalt phosphorus electroplating composition, comprising: a nickel salt; a cobalt salt; a phosphite-containing compound; and a multidentate chelating agent selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof.
 2. The nickel cobalt phosphorus electroplating composition of claim 1, wherein the multidentate chelating agent is triethylene tetraamine.
 3. The nickel cobalt phosphorus electroplating composition of claim 1, wherein the phosphite-containing compound is a sodium-free phosphite-containing compound selected from the group consisting of phosphorous acid, nickel phosphite, cobalt phosphite and combinations thereof.
 4. The nickel cobalt phosphorus electroplating composition of claim 3, wherein the phosphite-containing compound is phosphorous acid.
 5. The nickel cobalt phosphorus electroplating composition of claim 1, wherein the nickel salt is selected from the group consisting of nickel carbonate, nickel hydroxide, nickel oxide, and combinations thereof.
 6. The nickel cobalt phosphorus electroplating composition of claim 1, wherein the cobalt salt is selected from the group consisting of cobalt carbonate, cobalt hydroxide, cobalt oxide, and combinations thereof.
 7. An electroplating solution comprising a nickel cobalt phosphorus electroplating composition and water, the nickel cobalt phosphorous electroplating composition comprising: a nickel salt dissolved in the water to form nickel ions; a cobalt salt dissolved in the water to form cobalt ions; a phosphite-containing compound dissolved in the water to form phosphite ions; and a multidentate chelating agent dissolved in the water and selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof; wherein the electroplating solution has a pH value ranging from 0.2 to
 5. 8. The electroplating solution of claim 7, wherein the pH value ranges from 1.2 to
 2. 9. The electroplating solution of claim 7, wherein the pH value ranges from 1.5 to 1.9.
 10. The electroplating solution of claim 7, further comprising an electrolyte selected from the group consisting of phosphoric acid, sulfuric acid, hydrochloride, and combinations thereof.
 11. The electroplating solution of claim 10, wherein the concentration of the nickel ions ranges from 20 to 100 g/l, the concentration of the cobalt ions ranges from 0.5 to 15 g/l, the concentration of the phosphite ions ranges from 5 to 80 g/l, the concentration of the electrolyte ranges from 20 to 200 g/l, and the concentration of the multidentate chelating agent ranges from 20 to 200 g/l.
 12. The electroplating solution of claim 11, wherein the concentration of the nickel ions ranges from 40 to 70 g/l, the concentration of the cobalt ions ranges from 4 to 7 g/l, the concentration of the phosphite ions ranges from 20 to 40 g/l, the concentration of the electrolyte ranges from 100 to 140 g/l, and the concentration of the multidentate chelating agent ranges from 60 to 120 g/l.
 13. The electroplating solution of claim 12, wherein the concentration of the nickel ions is 55 g/l, the concentration of the cobalt ions is 5.5 g/l, the concentration of the phosphite ions is 30 g/l, the concentration of the electrolyte is 120 g/l, and the concentration of the multidentate chelating agent is 90 g/l.
 14. The electroplating solution of claim 10, wherein the electrolyte is phosphoric acid.
 15. The electroplating solution of claim 14, wherein the multidentate chelating agent is triethylene tetraamine.
 16. A method for treating a surface of a workpiece, comprising: placing the workpiece in an electroplating solution including a nickel cobalt phosphorus electroplating composition; and electroplating the workpiece in the electroplating solution under a current density so as to form a nickel cobalt phosphorus electrodeposit to on the surface of the workpiece, wherein the nickel cobalt phosphorus electroplating composition includes a nickel salt, a cobalt salt, a phosphite-containing compound, and a multidentate chelating agent selected from the group consisting of triethylene tetraamine, diethylene triamine, hydrazobenzene, and combinations thereof.
 17. The method of claim 16, wherein the nickel cobalt phosphorus electroplating composition further includes an electrolyte selected from the group consisting of phosphoric acid, sulfuric acid, hydrochloride, and combinations thereof.
 18. The method of claim 16, wherein the electroplating solution is maintained at a temperature ranging from 40° C. to 70° C. during the electroplating of the workpiece in the electroplating solution.
 19. The method of claim 18, wherein the electroplating solution is maintained at a temperature ranging from 50° C. to 60° C.
 20. The method of claim 16, wherein the current density ranges from 0.5 to 10 A/dm² during the electroplating of the workpiece in the electroplating solution.
 21. The method of claim 20, wherein the current density ranges from 1.5 to 6 A/dm².
 22. The method of claim 16, wherein the electroplating of the workpiece in the electroplating solution is conducted using an undissolvable anode.
 23. The method of claim 22, wherein the undissolvable anode is made from platinum titanium mesh.
 24. The method of claim 16, further comprising hot-working the electroplated workpiece after the electroplating of the workpiece in the electroplating solution.
 25. The method of claim 24, wherein the hot-working of the electroplated workpiece is conducted at a temperature ranging from 200° C. to 450°. 